In telephone subscriber loops it is necessary to supply direct current potential to subscriber equipment in order to effect communication. The current level supplied to the subscriber equipment must also be of sufficient magnitude to enable proper functioning of that equipment as well as the telephone company central office equipment. This current supply function is realized by employing a so-called battery feed circuit. Battery feed circuits typically include impedance elements commonly referred to as build-out resistors for limiting the current flow to the telephone loop in order to protect the central office battery, especially in ground start modes of operation. The current limiting impedance elements, however, operate also to limit the length of the subscriber loop connected to the battery feed circuit.
Consequently, it has become desirable to employ limiting elements or devices which exhibit a "high" impedance when a "low" impedance telephone loop is connected to the battery feed circuit and which exhibit a "low" impedance when a "high" impedance telephone loop is connected to the battery feed circuit.
A device which has long been employed for this purpose in telephone equipment is the so-called resistance lamp. The resistance of the lamp varies with variations in current. Thus, the lamp somewhat compensates for different length telephone loops connected to the battery feed circuit. One problem with the resistance lamp, however, is its relatively slow response to impedance changes or conditions of the telephone loop, for example, dial pulsing and the like. The response of the resistance lamp may be, for example, in the order of 15 to 20 seconds to go from an on to an off condition. Additionally, the lamps also have a relatively high impedance, therefore limiting the length of the subscriber loop which may be connected to the battery feed circuit.
Another device which has been employed as a battery feed current limiting elemment includes a non-linear resistance which varies in value in response to the magnitude of the loop current. As with the resistance lamp, the response of known non-linear resistance elements from an on to an off condition or vice versa is too slow. This and several other arrangements employed in attempts at minimizing attenuation of alternating signals are described in U.S. Pat. No. 3,187,104 issued June 1, 1965. However, the cited patent fails to address the problem of direct current attenuation caused by use of high resistance battery feed line build-out impedance elements.
Response time of the battery feed elements is also important to obtain improved performance during intervals of momentary changes of operational modes of the subscriber loop, for example, during dial pulsing, wink signaling and the like.
The lamp and other resistive elements heretofore employed for limiting current in battery feed circuits have relatively high resistance. Consequently, they were required to dissipate relatively "high" power necessitating use of "high" wattage elements. Therefore, it is also desirable to minimize the power dissipation in battery feed circuit elements in order to minimize the physical size of those elements and to improve overall efficiency.